China has barred a North Korean freighter from one of its ports and South Korea announced a crackdown on individuals and companies linked to Pyongyang's weapons program, stepping up sanctions against the isolated state.

North Korean general cargo ship Grand Karo arrived at Rizhao port in northeastern China a few days ago, but the port did not allow the ship to berth, said a person at the Rizhao Maritime Authority, declining to be identified because he was not authorized to speak to the media.

The ship is among 31 vessels blacklisted by China's Ministry of Transport after they were covered by harsher sanctions on North Korea that were approved by the U.N. Security Council last week.